It is commonly known to use solder alloy spheres for providing permanent connections between electronic devices and circuit boards. Typically, the composition of the spheres used is 63% tin and 37% lead, it being understood that all percentages used herein are by weight.
These discrete spheres of solder alloy are used as the connective medium in systems called ball grid arrays. Adhesion between the discrete spheres and the electronic device is achieved by printing a solder paste containing 63% tin and 37% lead onto the mounting pads of both the board and the electronic device. The solder paste and solder spheres are remelted on the electronic device to thereby attain sphere attachment thereto. Subsequently, die circuit board is positioned under the sphere laden device, the solder paste on the board being remelted, to effect a permanent bond between the device and the circuit board. Once the permanent bond between the device and circuit board has been effected, the device cannot be removed unless the solder is remelted which requires temperatures above 183.degree. C.
U.S. Pat. No. 5,324,569 issued Jun. 28, 1994 entitled Composite Transversely Plastic Interconnect for Microchip Carrier discloses microcarrier pin-out arrays used in integrated circuits in which transversely plastic interconnects join a microcarrier to a substrate of dissimilar material. Solder balls having a high melting temperature are coated with a thin (several microns thick) barrier metal layer to prevent reaction between the solder balls and low melting temperature solder paste and to keep the solder ball microsphere shape during reflow. However, the thin solder ball coating necessary to allow transverse plasticity in the balls to compensate for thermal coefficient of expansion mismatch between the microcarrier and the substrate renders the coated spheres easily deformable.
U.S. Pat. No. 5,093,986 issued Mar. 10. 1992 entitled "Method of Forming Bump Electrodes" discloses a method of producing bump electrodes which consist of adhesively securing metal balls to a circuit board by a conductive paste containing a metal such as Cu or Ag or an Ag--Pb alloy and firing at 850.degree. to 1000.degree. C. to fix the balls to the circuit board. The balls may be plated to render them rust resistant.
There exist important reasons which make it desirable to provide a ball grid array assembly connective medium which provides a non-permanent, or releasable connection between the device and the board. Exemplary reasons involve replacement of a failed device, or substitution by an upgraded model. Alternatively, it may be desirable to delay the final assembly of units until after shipment.
The prior art has contemplated the use of interconnecting pins functional as the connective medium to releasably attach the electronic device to the circuit board. Deleteriously, such methods have not only proved expensive but have been further found to be extremely fragile and difficult to implement.
Proprietary technology, which was, however, not developed by the present assignee, under investigation is exploring the viability of providing removably detachable or `pluggable` contacts between device and board which would provide easy engagement and disengagement therebetween. Such pluggable contacts typically comprise an array
U.S. Pat. No. 5,324,569 issued Jun. 28, 1994 entitled Composite Transversely Plastic Interconnect for Microchip Carrier discloses microcarrier pin-out arrays used in integrated circuits in which transversely plastic interconnects join a microcarrier to a substrate of dissimilar material. Solder balls having a high melting temperature are coated with a thin (several microns thick) barrier metal layer to prevent reaction between the solder balls and low melting temperature solder paste and to keep the solder ball microsphere shape during reflow. However, the thin solder ball coating necessary to allow transverse plasticity in the balls to compensate for thermal coefficient of expansion mismatch between the microcarrier and the substrate renders the coated spheres easily deformable.
There exist important reasons which make it desirable to provide a ball grid array assembly connective medium which provides a non-permanent, or releasable connection between the device and the board. Exemplary reasons involve replacement of a failed device, or substitution by an upgraded model. Alternatively, it may be desirable to delay the final assembly of units until after shipment.
The prior art has contemplated the use of interconnecting pins functional as the connective medium to releasably attach the electronic device to the circuit board. Deleteriously, such methods have not only proved expensive but have been further found to be extremely fragile and difficult to implement.
Proprietary technology, which was, however, not developed by the present assignee, under investigation is exploring the viability of providing removably detachable or `pluggable` contacts between device and board which would provide easy engagement and disengagement therebetween. Such pluggable contacts typically comprise an array of spheres soldered to the device and an array of spring-loaded sphere-receiving contacts mounted on an interconnecting member adapted to be permanently attached to the circuit board, said interconnecting member and board having electrical contact points at predetermined sites.
Thus, when the device must be replaced, for example because of failure or a desire to substitute a device having upgraded performance, the spheres and spring-loaded spheres receiving contacts are pulled apart, or conversely to interlock the device and board, are gently pressed together. The spheres must be hard and not deformable to maintain their sphericity and to avoid creep due to the pressure of top engagement contacts (FIG. 3). Transversely plastic spheres accordingly are not suitable.